Someone screams or a car alarm goes off. The hair on the back of your neck stands up. Your ears ache. You immediately start looking for a silent refuge.
Why do you react that way? Scientists now think they have an explanation for what it is about someone screaming that makes our brain think, 'Something bad is happening. Let's get out of here!'
In a study published Thursday in the journal Current Biology, a group of researchers at New York University, the University of Geneva and the Max Planck Institute for Empirical Aesthetics in Germany report that screams have a special quality called roughness. Simply put, roughness happens when the frequency or amplitude of a sound changes dramatically very quickly. (Frequency and amplitude are two defining characteristics of all kinds of waves, sound waves included.)
When this happens, "the ear is no longer able to 'break down' these temporal changes - such sounds are then perceived as rough and unpleasant," said David Poeppel, a neuroscientist at NYU and the lead author of the study, in a statement.
Other ways we use our voice to communicate, like straight-up talking or singing, don't have this audial fingerprint, which the authors say, points to the special niche that screams have in our communications toolbox.
Sound designers seem to have intuited this before scientists did. When researchers tested artificial sounds made by buzzers and horns, they found that these sounds also exhibit roughness. "The fact that roughness appears to be used in the design of artificial alarm signals in human culture, perhaps unwittingly, underlines both the perceptual salience and ecological relevance of rough sounds," the authors write in the paper.
Scientists had described roughness in the past, but the advance here is that they were able to show that only sounds we deem "alarming" have that roughness signature. They also report that sounds with a high roughness quotient are perceived as most alarming, and that the amygdala, a brain region that processes fear, was activated when people heard these types of sounds.
It's long been known that we use differences in frequency to decipher speech. For instance, we use certain frequencies to assign gender to speakers and how changes in intonation to attach meaning to what someone says. A person can say the same sentence with different intonations, and it can be interpreted as genuine, sarcastic, or straight-up mean. But until now, they didn't know what the "roughness" spectrum was for.
So next time you hear a buzzer going off or someone screaming at the top of their lungs, now you'll know why you find those screeching sounds so unpleasant.
Daniela Hernandez is a senior writer at Fusion. She likes science, robots, pugs, and coffee.